Cycles Render Lighting Glitch While Using Transparent Normals Map
From what I'm seeing in other discussions about this around the web (1, 2), Substance Designer embeds a height map in the alpha channel of the normal map PNG. In other words, the way Allegorithmic is using it in their normal maps "alpha" means height/bump, not transparency.

Theoretically this is convenient because only one PNG needs to be loaded into memory to provide both height and normal data. In practice some render engines, including Cycles will be confused by this, as Cycles quietly passes the alpha channel along with the RGB data. Cycles does not anticipate RGBA data being fed into a normal input socket - it expects RGB only.

The way to strip that out is by using a Separate RGB node, followed by a Combine RGB node. Then you can connect that RGB normal data as usual. 

If you will be doing this often you might want to Group these nodes together and save that group in your startup .blend.

Now what about that alpha channel containing the height data? Let's not let it go to waste. Here's how to use both height and normal data together in Cycles.

Remember to specify that the image texture is of the type "Non-Color Data".